The European Telecommunications Standardisation Institute (ETSI) is currently in the process of standardising a new set of protocols for mobile telecommunications systems. The set of protocols is known collectively as Universal Mobile Telecommunications System (UMTS).
FIG. 1 illustrates schematically a UMTS network 1 which comprises a core network 2 and a UMTS Terrestrial Radio Access Network (UTRAN) 3. The UTRAN 3 comprises a number of Radio Network Controllers (RNCs) 4, each of which is coupled to a set of neighbouring Base Transceiver Stations (BTSs) 5. BTSs are sometimes referred to as Node Bs. Each Node B 5 is responsible for a given geographical cell and the controlling RNC 4 is responsible for routing user and signalling data between that Node B 5 and the core network 2. All of the RNCs are coupled to one another. A general outline of the UTRAN 3 is given in Technical Specification TS 25.401 V3.2.0 of the 3rd Generation Partnership Project. FIG. 1 also illustrates a mobile terminal or User Equipment (UE) 6, a Serving GPRS Support Node (SGSN) 7 and a GPRS Gateway Support Node (GGSN) 8. The SGSN 7 and the GGSN 8 provide packet switched data services to the UE 6 via the UTRAN (with the GGSN being coupled to the Internet).
User data received at an RNC from the UTRAN core network is stored at a Radio Link Control (RLC) entity in one or more RLC buffers. User data generated at a UE is stored in RLC buffers of a peer RLC entity at the UE. User data (extracted from the RLC buffers) and signalling data is carried between an RNC and a UE using Radio Access Bearers (RABs). Typically, a UE is allocated one or more Radio Access Bearers (RABs) each of which is capable of carrying a flow of user or signalling data. RABs are mapped onto respective logical channels. At the Media Access Control (MAC) layer, a set of logical channels is mapped in turn onto a transport channel, of which there are three types:                a common transport channel which is shared by many different mobile terminals and which may extend in either the uplink or the downlink direction (one type of common channel is a Forward Access CHannel (FACH));        a “dedicated” transport channel (DCH) which is allocated to a single mobile terminal—DCHs are allocated in pairs of uplink and downlink channels; and        a downlink shared channel (DSCH) which is mapped to a small number of mobile terminals.        
Several transport channels are in turn mapped at the physical layer onto one or more physical channels (e.g. S-CCPCH or DPCH) for transmission over the air interface between a Node B and a UE.
FIG. 2 illustrates certain of the layers present at a UE, a Node B, and an RNC of a UMTS network. In particular, FIG. 2 illustrates that the MAC layer, present at the RNC and the UE, is split into a MAC-c layer and a MAC-d layer. UTRAN provides UEs with an “always on” connection. During periods of low activity, when perhaps only signalling information (or low level data transfer) is being exchanged between the UE and the network, the UE is allocated a common channel. However, following an increase in data volume, the network may decide to switch the connection from a common channel to a dedicated channel (and a downlink shared channel). For example, a decision may be made to switch from a FACH/RACH channel to a DCH. The decision to switch is made by the Radio Resource Manager (RRM) entity of the RNC.
It is envisaged that UMTS networks will to a large extent be used for carrying data traffic (and using the services of a SGSN and a GGSN). Most current applications which make use of packet switched data services use the Transport Control Protocol (TCP) in conjunction with Internet Protocol (IP)-TCP is used to provide a (connection-oriented) reliable service over the unreliable IP service. It can therefore be expected that the majority of data communications across a UMTS network will use TCP/IP.
When a TCP connection between peer hosts is initiated, TCP starts transmitting data at a relatively low rate. The transmission rate is slowly increased in order to avoid causing an overflow at routers of the IP network (which would result in the loss of packets and the need to resend these lost packets). The rate at which data can be transmitted is defined by two variables, cwnd and ssthresh.
The variable cwnd defines the number of data packets which the TCP sender can transmit before acknowledgement messages have been received from the TCP receiver for the previously sent block of cwnd data packets. As each acknowledgement is received for segments of that last block, a segment of the new block is sent. At the beginning of a communication, cwnd is set at a low value (e.g. 1 segment=512 bytes) and the system is in a “slow start” mode. Following receipt of the first acknowledgement from the receiver, cwnd is increased in size by 1 segment (to 2 segments). This is repeated following each acknowledgement for a packet (or for a small number of packets), resulting in an exponential opening of the congestion window. The variable ssthresh is initially set to some fixed level (e.g. 65535 bytes), and the slow start mode continues until cwnd>ssthresh. Thereafter, a “congestion avoidance” mode is entered during which cwnd is increased by just 1/cwnd each time a successful transmission acknowledgement is received. The variable cwnd has an upper limit defined either by the sender or by an advertisement message sent from the receiver.
If congestion occurs, indicated by a timeout (of a controlling timer at the sender) or by receipt of duplicate acknowledgements by the sender, ssthresh is set to one half of the previous value of cwnd. In the event that congestion was indicated by a timeout, cwnd is set to 1 whilst, in the event that congestion was indicated by a duplicate acknowledgement, cwnd remains unchanged. Thus, the slow start mode is continued until such time as the transmission rate (defined by cwnd) reaches half the rate which last caused congestion to occur. Thereafter, the congestion avoidance mode is entered.
Further background information on TCP/IP can be obtained from Stevens, “TCP/IP Illustrated”, Vol. 1.